Staffies are well known for being aggressive towards other dogs.
See
http://dogtime.com/dog-breeds/staffordshire-bull-terrier
Friendly with family: Five stars
Incredibly Kid Friendly Dogs: Five stars
Dog Friendly: One star!
[i]More info: Friendliness toward dogs and friendliness toward humans are two completely different things. Some dogs may attack or try to dominate other dogs even if they're love-bugs with people; others would rather play than fight; and some will turn tail and run. Breed isn't the only factor; dogs who lived with their littermates and mother until at least 6 to 8 weeks of age, and who spent lots of time playing with other dogs during puppyhood, are more likely to have good canine social skills.[i]
Friendly Toward Strangers: Five stars
Also worth noting:
Good For Novice Owners: One star
Easy to train: Two stars
Wanderlust potential: Four stars
So this event was almost inevitable in the circumstances.